The present invention concerns lead-free and cadmium-free glass compositions for glazing, enameling and decorating glass or glass-ceramics that contain high quartz and/or keatite solid solution crystals as a principal crystalline phase after crystallization and also concerns a method of making glass-ceramics provided with this glass composition.
Conventional glass-ceramics contain high quartz and/or keatite solid solution crystals as the principal crystalline phase, which are responsible for the low coefficient of thermal expansion. These glass-ceramics are transparent, translucent or opaque according to the crystalline phase and crystal size. The color is determined by the desired application and provided by means of oxide dyes. The principle fields of application of this type of glass-ceramic material with the lower thermal expansion include laboratory equipment, cooking vessels, fire protective glasses, chimney viewing windows and special heatable plates, e.g. cooking panels.
Decorative coatings are usually categorized as "glazes" and/or "enamels". The glazes generally include clear or colored glass (glass fluxes), while the enamels are coatings, which contain coloring, non-transparent materials, such as pigments. Colored inorganic compounds can be used as the pigments. The pigments usually may not react with glass flux or react only to a limited extent.
The glazes and enamels are used also for coating and improving glass-ceramics. Large-area coatings often are used for protection, covering or providing a pleasing appearance. Decorative glazes and enamels are used for writing for making a pleasing design or also for certain technical functions, such as in the case of display windows or markings of a cooking zone.
The glaze or enamel is burned-in at temperatures that are below the softening point of the article to be coated, so that the respective glass composition of the glaze or the enamel melts and forms a stable bond with the surface of the article. The burn-in temperature is generally below the softening point of the object to be coated so that no uncontrolled deformation can occur. The burning-in also acts to volatilize organic auxiliary substances, which, e.g., are used as suspension agents for the application of the glazes or enamels.
It is possible to find glazes or enamels with suitable coefficients of thermal expansion for glass or glass-ceramic coatings with thermal expansion coefficients of about 4.times.10.sup.-6 /K and greater. According to the state of the art it is desirable to use decorative coatings having a thermal expansion coefficient that is slightly less than that of the substrate to be coated. Because of that it is guaranteed that the glazes or the enamels turn out successfully on cooling after burning-in under pressure and exert no negative influences on the properties of the substrate, especially do not reduce the strength of the substrate. Stresses between the decorative coating and substrate build up when their thermal expansion coefficients do not match each other, so that cracks and tears arise, which can extend into the substrate material itself. The resulting stresses caused by the poor matching of the thermal expansion coefficients reduce the adherence of the coating. The decorative coating can immediately or in the course of time be worn off the substrate because of an especially poor match between the thermal expansion coefficients.
Problems arise in the current glaze or enamel decoration of glass-ceramics having a low thermal expansion based on high quartz or keatite solid solution crystals, which are made by thermal treatment, the so-called crystallization, of a suitable product glass. This type of glass-ceramics is characterized by a thermal expansion of less than 2.times.10.sup.-6 /K at temperatures between 20 and 700.degree. C. The decoration usually occurs at temperatures under 1200.degree. C. considering the softening point and the thermal resistance of these glass-ceramics. The burning-in of the glazes or enamels in the glass-ceramic material is preferably performed during the crystallization process, i.e. the decorative coatings are applied to the blank glass and burned-in during the crystallization. Currently no effective decorative coating with a matching thermal expansion coefficient is available for this type of glass-ceramic material with a low thermal expansion coefficient. Many attempts have been made to solve this problem of poor matching in order to avoid the occurrence of the serious disadvantages in the desired properties.
Especially the resulting reduction of the bending tensile strength occurring due to a complete-covering coating or heavily decorated surface is a serious disadvantage. The reduction of the bending tensile strength is based first on the unavoidable formation of stresses between the decoration and substrate due to the poor matching of thermal expansion coefficients and also because a certain amount of dissolving of the substrate by the decorative material and reaction at the surface is required for adherence of the decoration. It is possible to avoid the problem of reduction of bending tensile strength by using very light decorations, however complete-covering coatings for protection or heavy decorations, such as designs, are not possible. An average bending tensile strength of greater than 30 MPa is considered necessary for sufficient bending tensile strength in handling, formation and later use of the decorated glass-ceramics.
For example, the resulting stresses are reduced by using a decorative coating with reduced layer thickness, even with poor matching of the coefficients of thermal expansion. This means however that coloring properties (color coverage, color intensity) and protective action may be considerably reduced.
Glazes and enamels currently used for coating and/or for decoration of glass-ceramics with low thermal expansion coefficients frequently contain lead and also cadmium. Besides its beneficial effect in regard to a reduction of the burn-in temperature, the use of lead and cadmium provides a satisfactory adherence for the decorative coating, although the thermal expansion coefficient is in the size range of from 5.times.10.sup.-6 /K to up to 10.times.10.sup.-6 /K. The reason why this poor matching between the decorative coating and the glass-ceramic substrate can be tolerated, is ascribed to the plasticity of the remaining flux or enamel. Additives of lead and cadmium are thus beneficial for the strength of the decorated glass-ceramics and provide a comparatively good chemical resistance to weak acids and bases, such as commonly occur in the household, in food preparation or also as cleaning agents in industry.
In spite of these beneficial properties of the remaining coating today glazes and enamels may no longer contain lead and cadmium because of the undesirable toxicological properties of these elements. In the literature different paths are already being suggested to coat glass-ceramics with low thermal expansion coefficients without using lead or cadmium compounds.
In German Patent Document DE 42 41 411 A1 an attempt to solve the problem of poor matching between the glass-ceramic substrate and decorative coating by addition of chemically inert, optically inactive, elastic inorganic materials was described. This type of additive comprises, e.g., mica platelets, which provide a certain plasticity to the decorative coating. Adherent and wear-resistant decorative coatings were made using these additives. However the effect of these additives used to make these coatings on the color shade and reflectivity of the coating is not always desirable and therefore disadvantageous.
German Patent Document DE 42 01 286 C2 describes the use of lead-free and cadmium-free glass compositions for glazes and enamels used on glass and glass-ceramics with a thermal expansion coefficient of less than 5.0.times.10.sup.-6 /K. The composition described in this reference contains Li.sub.2 O, 0 to 12% by weight; MgO, 0 to 10% by weight; CaO, 3 to 18% by weight; B.sub.2 O.sub.3, 5 to 25% by weight; Al.sub.2 O.sub.3, 3 to 18% by weight; Na.sub.2 O, 3 to 18 % by weight; K.sub.2 O, 3 to 18% by weight; BaO, 0 to 12% by weight; SiO.sub.2, 25 to 55% by weight, TiO.sub.2, 0 to 5% by weight and ZrO.sub.2, 0 to &lt;3% by weight. The relatively high content of alkaline and alkaline earth metal oxides, here especially K.sub.2 O and CaO, provides the good adherence of the resulting decorative coatings. The high alkali and alkaline earth oxide content effects chemical resistance to acids and strength of the glass-ceramic material disadvantageously.
U.S. Pat. No. 5,326,728 claims a lead-free frit containing SiO.sub.2, 35 to 50% by weight; B.sub.2 O.sub.3, 23 to 30% by weight; Al.sub.2 O.sub.3, 10 to 22% by weight; Li.sub.2 O, 1 to 3% by weight; Na.sub.2 O, 0 to 3% by weight; K.sub.2 O, 2 to 5% by weight; CaO, 1 to 5% by weight; TiO.sub.2, 0 to 2% by weight and ZrO.sub.2, 0 to 5% by weight, wherein the total sum of the Li.sub.2 O, Na.sub.2 O and K.sub.2 O present is less than 8% by weight and the total amount of CaO, MgO, ZnO, BaO and SrO present is less than 7% by weight. Additives of K.sub.2 O and CaO are required in this composition in order to achieve sufficient adherence in spite of the poor matching. For high requirements for acid resistance and strength of the decorative glass-ceramic coating these compositions are however often insufficient.
Especially the presence of K.sub.2 O in amounts of&gt;2% by weight has proven to be extremely deleterious for the bending tensile strength of the decorated glass-ceramic material. The potassium atom is very mobile during burning-in of the decoration and reaches the vicinity of the surface layer between the glass ceramic substrate and the decorative coating. This produces additional stresses that definitely reduce the bending tensile strength. The presence of K.sub.2 O in amounts of 2% by weight and larger occurs indeed results in the use of lighter decorative embodiments. Complete-covering coatings for protection or heavy decorative layers desirable for esthetically pleasing designs are thus only obtained to a limited extent.